The invention relates to a decorative paper sheet made impregnable by a thermosetting resin. This sheet is used in the manufacture of laminated decorative panels or moulded section. The invention also relates to the decorative sheets impregnated with a thermosetting resin as well as to the laminated decorative panels or moulded section which include it, and to the processes for manufacturing the sheets.
Laminated decorative panels or moulded section (also called xe2x80x9claminatesxe2x80x9d) have been used for many years as materials in dwellings and in commercial and industrial premises. Typical applications of such laminates are coverings for furniture, tabletops, chairs and other articles or floor coverings such as, in particular, coverings imitating a parquet floor.
There are two main types of decorative xe2x80x9claminatesxe2x80x9d, namely so-called high-pressure xe2x80x9claminatesxe2x80x9d and so-called low-pressure xe2x80x9claminatesxe2x80x9d.
The so-called high-pressure decorative laminates are produced from a core consisting of resin-impregnated sheets. These sheets are generally made of kraft paper and have been impregnated with a thermosetting resin, usually a phenolic resin.
After the sheets have been impregnated with resin, they are dried, cut and then stacked on top of one another. The number of sheets in the stack depends on the applications and varies between three and nine, but may be greater than this.
Next, the stack of sheets forming the core is placed on a decorative sheet. In general, a protective covering sheet, called an xe2x80x9coverlayxe2x80x9d, devoid of any pattern and transparent in the final laminate, is placed on top of the decorative sheet in order to improve the abrasion resistance of the laminate.
Next, the impregnated sheets are placed in a laminating press, the platens of which are provided with a metal sheet giving the laminate its surface finish. The stack is then densified by heating, at a temperature of about 110xc2x0 C. to 170xc2x0 C., and by pressing, at a pressure of about 5.5 MPa to 11 MPa, for approximately 25 to 60 minutes in order to obtain a unitary structure.
Next, this structure is fixed to a base support, for example by adhesively bonding it to the said support, such as a particleboard, especially a wood chipboard.
The so-called low-pressure decorative laminates are produced using only a decorative sheet impregnated with thermosetting resin, and optionally an overlay sheet, which are laminated directly to the base support such as a board during a short cycle, the temperature being about 160 to 170xc2x0 C. and the pressure 1.25 MPa to 3 MPa.
The impregnable decorative sheet used for the manufacture of laminates is generally a sheet of paper produced on a papermaking machine and which includes cellulose fibres and optionally synthetic fibres, the cellulose fibres being split between from 40 to 100%, preferably from 80 to 100%, by weight of short fibres and from 0 to 60%, preferably from 0 to 20%, by weight of long fibres, from 0.2 to 1%, preferably from 0.4 to 0.5%, by dry weight with respect to the sheet of a wet-strength agent and from 5 to 50% by dry weight with respect to the sheet of decorative particles, for example iridescent pigments, and/or of pigmentary colorants or organic dyes, and/or of opacifying fillers such as titanium dioxide, especially of the rutile type, the said opacifying filler such as titanium dioxide being in quantities of preferably at least 15% and generally between approximately 15 and 40% with respect to the weight of the sheet. It may also contain other additives usually employed in papermaking and, in particular, retention agents or specific agents such as alkaline products allowing characteristics such as post-forming to be controlled.
These decorative sheets do not include a sizing agent, nor are subjected to any surface treatment, as they must be highly absorbent with respect to the thermosetting resin with which they will be impregnated.
Moreover, a decoration may be printed on this sheet, for example a decoration imitating wood or any other fancy decoration.
Next, this sheet is impregnated with a thermosetting, but thermally stable (non-yellowing), resin, usually with melamine-formaldehyde resins or urea-formaldehyde resins, or sometimes with benzoguanamine-formaldehyde resins or unsaturated polyester resins. In a second step, the impregnated sheet is heated and the resin is partially crosslinked (thermally cured) so that the resin is no longer in a tacky state and the sheet can be handled. Such a decorative sheet impregnated with partially cured resin is called, in the art, xe2x80x9cdecoration filmxe2x80x9d or xe2x80x9cdecorative filmxe2x80x9d or xe2x80x9cmelamine-resin filmxe2x80x9d.
This second step is generally carried out by heating the sheet at temperatures of approximately 110 to 140xc2x0 C. and is controlled, so that the resin during the final lamination of the decoration film flows correctly into the sheet, by measuring the content of volatiles remaining in the decoration film since the latter then includes a certain percentagexe2x80x94about 5 to 8%xe2x80x94of volatile products (water, being the solvent for the resin, water resulting from the chemical condensation of the resin, residual formaldehyde, other residual products, etc). These volatiles represent compounds which will be removed during the complete curing of the resin, during the lamination of the decoration film.
The resin, once it has been thermally cured completely, will provide, after lamination, the surface resistance of the final laminate (abrasion resistance, resistance to soiling, to steam and to chemicals, such as solvents, acids and bases, etc).
Moreover, this sheet, once laminated, must have a very high degree of lightfastness since it is exposed almost permanently to light radiation because of its use as a surface covering; the compounds of which it is composed must therefore be selected so as to obtain this lightfastness, which is preferably greater than or equal to 6 on the scale of blues according to the ISO 4586-2.16 standard.
In addition, this sheet must allow an opaque decoration film to be obtained after lamination since it is important not to be able to see, through the said sheet, the base support and/or the sheets of kraft paper on which the impregnated decorative sheet has been laminated, so that there is no interference with the decoration of the sheet. It is therefore necessary to have a decorative sheet which is as opaque as possible.
In the case of a white decoration, for which a very white opacifying filler such as titanium dioxide is used, one is then forced to use large quantities (approximately 40% by weight with respect to the sheet) of fillers such as titanium dioxide in order to obtain this opacity. In fact, after impregnation and lamination, it is only the titanium dioxide which provides the opacity since, because cellulose has a refractive index close to that of the resin, the cellulose fibres are rendered transparent, this being all the more so the greater the amount of resin.
It is therefore necessary to incorporate as much filler, such as titanium dioxide, as possible, but this is expensive and degrades the mechanical properties of the sheet.
In order to reduce the cost of the decoration film, it is sought to minimize the amount of resin absorbed by the sheet while still maintaining the mechanical and surface-resistance properties of the decorative laminate that the thermally cured resin gives it.
This problem is especially important in the case of low-pressure laminated decorative panels or moulded section since the impregnated decorative sheet is directly laminated to the support board.
In Patent Application EP 677,401, these problems were dealt with by proposing a decorative sheet which includes a sizing agent and by preferably creating a thermosetting-resin absorption gradient so that the upper part of the sheet is richer in resin than the lower part internal to the laminate. The sizing agent, because of its hydrophobic effect, prevents the impregnation resin from completely penetrating into the sheet. The sizing agent is added in the bulk or to the surface, and in a differential manner, or by forming two plies, the lower ply having the sizing agent.
However, it has been noted that this latter approach is difficult to implement, does not always allow uniform flow of the resin into the sheet and can lead to defects in the appearance of the laminated decorative panel, especially in the case of low-pressure laminated decorative panels.
The invention aims to solve these problems and the object of the invention is therefore to provide a decorative paper sheet that can be used in decorative laminates, the said sheet resulting in a high opacity of the final decorative laminate and allowing a decorative laminate to be obtained without any defects in its appearance.
The Applicant has found that the object of the invention is achieved if the paper sheet has a 60-second Cobb water absorption value or COBB60, determined according to the ISO 535 standard, of less than the grammage of the said sheet by at most 40%, whereas the sheets according to the prior art and without a sizing agent, these being composed almost essentially of cellulose fibres, always have a 60-second Cobb water absorption value equal to or greater than their grammage.
Thus, it seems that the wettability of the sheet is maintained and therefore that the thermosetting resin is distributed uniformly in the sheet and, moreover, that the capacity of the sheet to absorb a thermosetting resin is reduced.
More specifically, the invention provides a decorative paper sheet, made impregnable by a thermosetting resin, comprising from 5 to 50% by dry weight with respect to the sheet of decorative particles and/or of pigmentary colorants or organic dyes and/or of opacifying fillers, particularly titanium dioxide, characterized in that the 60-second Cobb water absorption value of the said sheet, determined according to the ISO 535 standard (water, 1 minute, 23xc2x0 C.) is less than the grammage of the said sheet by at most 40% and, more particularly, by at least 5%.
Preferably, the COBB60 value is from 10 to 35% less than the grammage of the said sheet.
In one advantageous embodiment, the sheet comprises a composition of at least one polymer having a hydrophilic character. The hydrophilic character of the composition may stem from the chemical nature of a said polymer itself or from emulsifying agents when the said polymer is a water-insoluble polymer used in the form of an aqueous dispersion.
The hydrophilic character of the said composition or of the said polymer is therefore controlled (neither too high, nor too low) so as to reduce the 60-second Cobb water absorption value so as to be within the abovementioned limits, namely a reduction of at most 40% with respect to the grammage of the said sheet.
The hydrophilic character of the said composition or of the said polymer also makes it possible to reduce the 60-second Cobb water absorption value of the sheet, determined according to the ISO 535 standard (water, 1 minute, 23xc2x0 C.), and to do so by at most 35% with respect to the 60-second Cobb water absorption value of the same sheet not containing the said composition, and preferably by at least 5%.
The present invention therefore also provides a decorative paper sheet, made impregnable by a thermosetting resin, comprising from 5 to 50% of decorative particles and/or of pigmentary colorants and/or organic dyes or of opacifying fillers, particularly titanium dioxide, by dry weight with respect to the sheet, characterized in that it comprises a composition of at least one polymer having a hydrophilic character and the 60-second Cobb water absorption value of the sheet, determined according to the ISO 535 standard, is at most 35% less than the 60-second Cobb water absorption value of the same sheet not containing said polymer(s).
Preferably, the grammage of the sheet containing the said polymeric composition is from 50 to 150 g/m2, preferably from 60 to 100 g/m2.
According to a particular case, the sheet comprises from 4 to 20% by weight of said polymer(s) with respect to the sheet, especially from 2 to 10 g/m2 by dry weight of said polymer(s).
Preferably, the said composition comprises a water-insoluble polymer in aqueous dispersion.
Also preferably, the said water-insoluble polymer is a polymer having a hydrophilic character. As water-insoluble polymer having a hydrophilic character, mention may be made of polymers chosen from vinyl acetate polymers and particularly vinyl acetate/butyl acetate copolymers, vinyl acetate/ethylene copolymers or acrylic ester copolymers, particularly ethyl acetate/acrylonitrile/methacrylate copolymers, or else blends thereof. Other water-insoluble polymers having a hydrophilic character, which are known as binders, although according to the invention these do not involve the desired technical effect, may be compatible with the application according to the present invention, especially if they do not impair the good lightfastness of the decorative laminate, this preferably being greater than or equal to 6 on the scale of blues according to the ISO 4586-2.16 standard.
A polymeric composition comprising, as a mixture, an aqueous dispersion of a water-insoluble polymer and an aqueous solution of a water-soluble polymer, especially a poly(vinyl alcohol) polymer, may also be used.
More particularly, the composition may comprise from 80 to 95% by dry weight of a water-insoluble polymer and from 5 to 20% by dry weight of water-soluble polymer. The said water-soluble polymer may help to improve the uniformity in the appearance and in the steam resistance of the final panel and to reduce the time required to impregnate the sheet with the thermosetting resin.
The said polymer is advantageously introduced into the sheet by an impregnation process, especially using a size press containing the said polymer in aqueous medium. Any other means of application allowing good penetration of the film by the polymer may also be employed. The treatment composition may also contain standard papermaking additives, especially viscosity-control agents and antifoam agents.
The solids content and the viscosity of the composition may be controlled by those skilled in the art depending on the means of application used and on the amount of the said polymer to be introduced into the sheet.
Optionally, the said polymer may be added, in the bulk, in the pulp chest of a papermaking machine.
Preferably, the opacifying fillers are titanium dioxide pigments and are present in an amount ranging from 20 to 45%, more particularly from 30 to 40% by dry weight of the sheet. Other white fillers may be used, such as kaolin or talc, either as a complement to the titanium dioxide or as a mixture with pigmentary colorants.
In one embodiment, the sheet of paper according to the invention comprises:
cellulose fibres and, optionally, synthetic fibres, the cellulose fibres being split between from 40 to 100%, preferably from 80 to 100%, by weight of short fibres and from 0 to 60%, preferably from 0 to 20%, by weight of long fibres,
from 0.2 to 1%, preferably from 0.4 to 0.5%, by dry weight with respect to the weight of the sheet of a wet-strength agent.
The invention also provides a decorative paper sheet impregnated with a partially crosslinked thermosetting resin (decoration film) which is characterized in that it includes the said decorative sheet with the said polymer and that it contains at most 50% and preferably at least 40%, even more preferably at least 45%, by weight of thermosetting resin with respect to the weight of the resin-impregnated sheet, volatile compounds included.
The impregnated decorative sheet according to the invention may also be characterized in that it contains at most 50% and preferably at least 40%, even more preferably at least 45%, by weight of thermosetting resin with respect to the weight of the resin-impregnated sheet, volatile compounds included, and an amount of titanium dioxide of less than 40%, preferably less than 35%, by dry weight of the sheet and having an opacity identical to that of a decorative sheet of the same grammage before impregnation by the said resin, and containing an amount of titanium dioxide of at least 40% by dry weight of the sheet and more than 50% of said resin with respect to the weight of the resin-impregnated sheet, volatile compounds included.
In particular, the thermosetting resin is chosen from among melamine-formaldehyde resins and urea-formaldehyde resins or from among benzoguanamine-formaldehyde resins and unsaturated polyester resins, or blends thereof.
The subject of the present invention is also a process for manufacturing, by wet route, the said decorative sheet, which is characterized in that it comprises the following steps:
a sheet is formed on a papermaking machine from an aqueous suspension of cellulose fibres and, optionally, of synthetic fibres, and from 5 to 50% by dry weight with respect to the sheet of decorative particles and/or of pigmentary colorants or organic dyes and/or of opacifying fillers, particularly titanium dioxide, and optionally of other additives usually employed in these sheets,
it is drained and, optionally, dried,
the sheet formed is impregnated, using a size press, with a composition in aqueous medium containing one or more of the said polymers,
the sheet is drained and dried.
According to one particular case, the process comprises the following steps:
a sheet is formed on a papermaking machine from an aqueous suspension of cellulose fibres and optionally of synthetic fibres, these fibres being split between from 40 to 100%, preferably from 80 to 100%, of short fibres and from 0 to 60%, preferably from 0 to 20%, of long fibres, from 0.2 to 1%, preferably from 0.4 to 0.5%, by dry weight with respect to the sheet of a wet-strength agent and from 5 to 50% by dry weight with respect to the sheet of decorative particles and/or of pigmentary colorants or organic dyes and/or of opacifying fillers, particularly titanium dioxide, and optionally of other additives usually employed in these sheets,
it is drained and, optionally, dried,
the sheet formed is impregnated, using a size press, with from 4 to 20% by dry weight, especially from 2 to 10 g/m2 by dry weight, with respect to the sheet of a composition in aqueous medium containing one or more of the said polymers, the solids content of the composition being especially between 5 and 20% by weight,
the sheet is drained and dried.
In one particular case, the invention also provides a process for manufacturing a decorative sheet impregnated with a partially crosslinked thermosetting resin (decorative film) which is characterized in that a paper sheet as described above is used, in that the said sheet is impregnated with a thermosetting resin and in that the resin is partially crosslinked, the content of volatile compounds being between 5 and 8% by weight of the sheet.
The subject of the invention is also a laminated decorative panel or profile which is characterized in that it includes, as decorative sheet, a sheet as described above.
In particular, this laminated decorative panel or profile is a low-pressure laminate which may be produced conventionally, as described in the introduction.